A brief, novice Arduino hardware sketch to

hardware-sketch.ino

#include <Stepper.h>

const int steps_per_revolution = 200;
const int step_pin = 3;
const int direction_pin = 4;
const int end_stop_pin = 5;
const int steps_per_motion = steps_per_revolution / 2;
const int motor_speed = 50;
const int led_pin = 13;

String current_command = ""; 
bool is_ready = true;
bool next_command_ready = false;
int end_stop_state = 0;
unsigned long previousMillis = 0; 
long interval = 50;

Stepper stepController(steps_per_revolution, step_pin, direction_pin);

void setup() {
  Serial.begin(9600);
  Serial.setTimeout(50);
  pinMode(led_pin, OUTPUT);
  pinMode(end_stop_pin, INPUT);
  stepController.setSpeed(motor_speed);
}

void loop() {
  unsigned long current_millis = millis();
  if (next_command_ready) {
    if (current_millis - previous_millis > interval) {
      current_command.trim();
      if (current_command.startsWith("STOPPED_TOUCHING_CUP")) {
        removeFingerFromCup();
      }
      if (current_command.startsWith("BEGIN_TOUCHING_CUP")) {
        putFingerOnCup();
      }
      current_command = "";
      next_command_ready = false;
    }
  }
}

void putFingerOnCup() {
  digitalWrite(led_pin, HIGH);
  stepController.step(-steps_per_motion);
}

void removeFingerFromCup() {
  digitalWrite(led_pin, LOW);
  stepController.step(steps_per_motion);
}

void serialEvent() {
  while (Serial.available()) {
    char chunk = (char)Serial.read();
    current_command += chunk;
    if (chunk== '\n') {
      next_command_ready = true;
    }
  }
}